A distributed feedback (DFB) laser may be disposed within a semiconductor device such as a laser diode (LD). An active region of the DFB LD may comprise a diffraction grating to provide optical feedback for the laser over a narrow wavelength band, which may be selected according to the pitch with which the grating is fabricated. The narrow output spectrum characteristic of DFB lasers gave rise to their usefulness in optical communications, in which information is exchanged over networks of optical fiber and other transmission media. Conventional semiconductor distributed feedback lasers have been used as light sources for powering the fiber-optic based internet and related networks since the mid-1970s. Since then, volumes of network traffic have exploded, along with concomitant demand for higher bandwidth and increased data rates.
For example, the explosive growth of the internet over the last 20 years has created a geometrically increasing demand for bandwidth. Existing communication approaches typically meet this bandwidth demand by an optical fiber network with multiple channels. Using dense wavelength division multiplexing (DWDM) techniques, each of the multiple channels comprises an optical wavelength different from an optical wavelength of each of the other channels.
Information is transmitted over DWDM networks at 1 bit per pulse by modulating the intensity of the light source (e.g., on/off keying) at speeds up to 10 Gb/s. The upper data rate is typically limited by optical impairments, which are induced or introduced by the optical fiber transmission media. The full utilization of the available number of channels in the optical spectrum along with the bound on modulation rates has instigated the search for alternative information transmission schemes to meet the ever increasing bandwidth demand.
The growing demand for transmitting information at ever-higher data rates has led to the development of coherent communication, in which information is encoded on an optical wave using principally a modulation of its phase. Quantum-based limitations related to their inherent phase or temporal coherence characteristics limit the phase stability of conventional DFB lasers. The channel capacity of conventional DFB lasers is thus insufficient for handling the demands imposed by the migration of networks to coherent communication.
Improved coherence has thus been long sought in semiconductor DFB lasers. Previous approaches have used elongation of the laser cavities, multiple phase-shifts for the engineering of longitudinal modes therein, optimization of the active laser medium, e.g. strained QW (quantum well), and wavelength detuning.
The spectral linewidths achieved using such techniques in commercial and other conventional or state of the art lasers however remain persistently high. For example, spectral linewidths of conventional DFB lasers remains above 100 kHz, and this linewidth value itself reflects a narrowness that may be attained only using high pump currents. Moreover, linewidths at this level remain too high to satisfy the demands presented by multi-phase coherent communication and other useful applications.
Some fiber based lasers, which have linewidths below 1 kHz, and external cavity lasers (ECL), which have linewidths below 10 kHz, have high coherence characteristics. However, they typically have bulky and complex structures, which render them incompatible with the physical scaling demands of growing networks.
Contemporary optical communication networks are powered extensively by semiconductor lasers, including conventional DFB LDs, because of the benefits of their small size, high power output, high efficiency, low cost, and potential integration opportunities with associated electronic circuits. Due, however, to their significant phase noise characteristics, primarily of intrinsic quantum mechanical origin, conventional semiconductor lasers, including typical conventional DFB LDs, are typically incapable of meeting the stringent spectral purity requirements to ultra-high-speed communication networks.
A semiconductor distributed feedback laser capable of handling the requirements imposed by coherent communication networks, without using DWDM would thus be useful. A distributed feedback laser free of the quantum-based phase or temporal coherence characteristics inherent in conventional DFB lasers would also thus be useful. It would further be useful to improve significantly the phase or temporal coherence characteristics and the channel capacity of a distributed feedback laser relative to conventional DFB LDs.
Approaches described in this section may, but not necessarily, have been conceived or pursued previously. Unless otherwise indicated, it should not be assumed that any approaches discussed above include any alleged prior art merely by any such discussion. Not dissimilarly, any issues discussed in relation to any of these approaches should not be assumed to have been recognized in any alleged prior art merely based on any such discussion above.